6458
Langmuir 2002, 18, 6458-6461
Preparation and Rheological Properties of Emulsion Gels Anne Koenig, Pascal He´braud, and Patrick Perrin* Laboratoire de Physico-Chimie Macromole´ culaire, UMR 7615, Ecole Supe´ rieure de Physique et Chimie de Paris (ESPCI), CNRS, Universite´ Pierre et Marie Curie (UPMC), 10, rue Vauquelin, 75005 Paris, France
further, no interaction and strong interactions between the droplets and the gel matrix are expected in the former and in the latter case, respectively. Second, we investigated the elastic properties of our system and compared our experimental results to those obtained using a mean field theoretical model that describes the rheological behavior of spherical deformable droplets dispersed in a continuous medium.11 Experimental Section
Received January 18, 2002. In Final Form: April 18, 2002
Introduction Simple emulsions consist of droplets of one liquid dispersed in a second immiscible liquid. As the droplet concentration increases, they exhibit a transition from a viscous fluid to an elastic solid at dispersed phase concentrations near the random close-packing concentration (cpc).1-3 The preparation of emulsions with droplets suspended in a viscoelastic matrix is also an interesting route leading to the formation of solidlike emulsions, and hence to materials with original textures.4 Advantageously, this method allows the preparation of elastic liquid-liquid dispersions with droplet concentrations lower than cpc. Recently, we studied the stability and rheological behavior of direct emulsion systems in which oil droplets were trapped into an aqueous physical gel matrix consisting of associative polymer molecules.5,6 Also, we reported on the preparation of a covalent gel matrix to immobilize the crystalline structure of ordered monodisperse direct emulsions stabilized by amphiphilic polyelectrolytes.7 In the latter case, the droplets are then embedded into a chemical gel. The viscoelastic properties of these materials depend not only on that of the gel matrix and on the droplet concentration but also on the type of the droplet-matrix interactions as shown by van Vliet et al.8 and by Chen et al.9,10 in the case of protein emulsion gel systems. Since the droplet-matrix interfacial properties play a crucial role in the mechanics of emulsion gel systems, it is of importance to study the dependence of their viscoelastic properties as a function of their tunable interactions between the droplets and the suspending medium. First, we thus reported in this paper on the preparation of a simple direct emulsion gel system for which the interfacial interactions could be adjusted. On one hand, oil droplets were dispersed in a gel matrix composed of cross-linked linear poly(sodium acrylate) (PAANa) chains. On the other hand, we also prepared emulsions with oil droplets dispersed in a matrix made of cross-linked linear hydrophobically modified poly(sodium acrylate) (HMPAANa) molecules. As discussed * Corresponding author. Postal address: ESPCI-LPM, 10, rue Vauquelin, 75005 Paris, France. (1) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160-175. (2) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427-437. (3) Mason, T. G.; Bibette, J.; Weitz, D. A. Phys. Rev. E 1995, 75, 2051-2054. (4) Gallegos, C.; Franco, J. M. Curr. Opin. Colloid Interface Sci. 1999, 4, 288-293. (5) Perrin, P.; Lafuma, F.; Audebert, R. Prog. Colloid Polym. Sci. 1997, 105, 228-238. (6) Perrin, P.; Devaux, N.; Sergot, P.; Lequeux, F. Langmuir 2001, 17, 2656-2663. (7) Perrin, P. Langmuir 2000, 16, 4774-4778. (8) van Vliet, T. Colloid Polym. Sci. 1988, 266, 518-524. (9) Chen, J.; Dickinson, J. J. Texture Stud. 1998, 29, 285-304. (10) Chen, J.; Dickinson, E. J. Agric. Food Chem. 1998, 46, 91-97.
Materials. Linear PAANa was synthesized by aqueous radical polymerization using acrylic acid (Fluka) and the redox couple ammonium persulfate/sodium metabisulfite (Prolabo) as the initiator.12 Size exclusion chromatography measurements were performed on the synthesized polymer, giving a weight-average molecular weight of about 150 000 g/mol and a polydispersity index of 2.1. The PAANa polymer was then used to synthesize HMPAANa according to the method described by Wang et al.13 The chemically grafted hydrophobic groups (3 mol % of ndodecylamine (Fluka)) are randomly distributed along the polymer backbone.13 Emulsion gels were prepared using doubledistilled deionized water (Milli-Q system from Millipore) and n-dodecane (Prolabo). Gelation was performed using hexamethylene diamine (Aldrich) as a cross-linking agent and EDC (N′-3-dimethylaminopropyl-N-ethylcarbodiimide hydrochloride, Aldrich), a water soluble carbodiimide, as a coupling agent. Methods. We used a Heidolph Diax 900 rotor-stator type of homogenizer for the preparation of the HMPAANa emulsion gels, whereas an ultrasonic microprobe (Bioblock Scientific Vibracell) was used to prepare the PAANa emulsion gels. Small-deformation viscoelasticity was investigated by dynamic oscillatory rheometry using a controlled stress rheometer (Rheometrics RFS II) equipped with a plate-plate geometry. Gel samples were prepared in the shape of cylinders with diameter and thickness of 8 mm and 5 mm, respectively. We checked that neither evaporation nor slipping at interfaces occurred during the measurements. A drop tensiometer (Tracker from ITConcept) was used to measure the oil/aqueous polymer solution interfacial tension and elasticity at 25 °C. Finally, the distribution of droplet hydrodynamic radii was determined using dynamic light scattering.
Results and Discussion Aqueous Polymer Gels: Effect of Polymer Concentration and Diamine Concentration. The rheology of emulsion gels has been investigated by Chen and Dickinson14 using cross-linked proteins as the matrix. They changed the interface properties of the oil droplets by adding nonionic surfactants. In our work, we used simple synthetic polymer gels having tunable surface properties with no need of surfactant addition. Hence, it is worthwhile to give a thorough description of the sample preparation. We first present the preparation of the aqueous polymer gels. Aqueous PAANa or HMPAANa solutions were first prepared by dissolving the appropriate amount of polymer in water. Solutions were gently stirred for 24 h to ensure a good dissolution of the polyelectrolyte. The polymer concentrations (Cp) are given in weight of polymer per weight of solution. To incorporate the cross-linker, aliquots of a 10% w/w hexamethylene diamine aqueous solution (pH ) 8) were added to the polymer solutions. The hexamethylene diamine concentrations (Ca) are expressed (11) Palierne, J. F. Rheol. Acta 1990, 29, 204-214. (12) Bokias, G.; Durand, A.; Hourdet, D. Macromol. Chem. Phys. 1998, 199, 1387-1392. (13) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577-582. (14) Chen, J.; Dickinson, E. Colloids Surf., B 1999, 12, 373-381.
10.1021/la0200663 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/12/2002
Notes
Langmuir, Vol. 18, No. 16, 2002 6459 Scheme 1
in mole of amine per mole of monomer repeat unit. In the case of the hydrophobically modified polymer, the HMPAANa average monomer repeat unit was used in the calculation of the amine concentration. The polymer and amine mixture was stirred for 4 h, and the pH was readjusted at 8 when necessary. Finally, the coupling agent EDC dissolved in water (30% w/w) was added to the aqueous mixture under stirring in order to connect the polymer chains, and hence to obtain either PAANa or HMPAANa covalent gels. A few minutes later, gelation was completed. The amount of the EDC coupling agent was calculated so that the EDC/amine molar ratio was equal to 3. The same ratio will be used for the preparation of the emulsion gels. The chemical reaction used to link the polymer chains thus reads as in Scheme 1. The reaction was already used to graft polyether chains on poly(sodium acrylate).15 The effects of the polymer concentration (Cp) and diamine concentration (Ca) on the gelation of both PAANa and HMPAANa aqueous polymer solutions were first studied. Figure 1 gives a qualitative idea of the values of Cp and Ca required for the formation of a gel sample. The diagram (Figure 1a) was built according to the visual observation that viscous liquid samples were able to flow under gravity while gel samples were not. As expected, the solidlike sample domain of both PAANa and HMPAANa gels increases with increasing Cp or Ca (Figure 1a). This preliminary study allows us to determine the conditions under which elastic gels are actually obtained. Elastic moduli of both types of gel were measured as a function of the amine concentration at a polymer concentration of 6% (Figure 1b). In the rest of the report, Cp and Ca are kept constant and equal to 6% and 10%, respectively. Under these conditions, it is clear that both types of gels are elastic. The measured value of the elastic modulus of the HMPAANa gel (4680 Pa) was found to be larger than that of the PAANa gel (1470 Pa). The higher elastic modulus of the hydrophobically modified polymer gel can be explained by the presence of physical bounds, due to the formation of intermolecular hydrophobic aggregates at such a high polymer concentration (Cp ) 6%),5,16 in addition to the covalent links resulting from the cross-linking chemical reaction described above. Emulsion Gel Preparation. The preparation of emulsion gels formulated with HMPAANa is now presented. Aqueous mixtures of HMPAANa and hexameth-
ylene diamine were first prepared as described above. n-Dodecane was then added to the aqueous solution, and the two phases were mixed at ambient temperature using the homogenizer for 5 min at 26 000 rpm. Right after the emulsion sample preparation, EDC in water solution (30% w/w) was added to the n-dodecane in water emulsion to gel the external phase of the emulsion. The whole mixture was then stirred for 2 min before it was quickly poured into a mould to obtain emulsion gels with flat surfaces. The samples were then removed from the mould 24 h later and studied by rheology in a plate-plate geometry with a gap spacing equal to the sample thickness. The rheological measurements are discussed below in a separate paragraph. Note that the chemical reaction in
(15) Hourdet, H.; L’Alloret F. F.; Audebert, R. Polymer 1997, 38, 2535-2547. (16) Wang, T. K.; Iliopoulos, I.; Audebert, R. In Water-Soluble Polymers: Synthesis, Solution Properties and Applications; Shalaby, S. W., McCormick, C. L., Butler, G. B., Eds.; ACS Symposium Series 467; American Chemical Society: Washington, DC, 1991; p 218.
Figure 1. (a) Effect of the polymer concentration (Cp) and diamine concentration (Ca) on the gelation of both the PAANa and HMPAANa hydrogels. (b) Variation of G′ with the amine concentration at constant polymer concentration (Cp ) 6%): (b) HMPAANa gels; (2) PAANa gels.
(a)
(b)
6460
Langmuir, Vol. 18, No. 16, 2002
Notes
Figure 2. Variation of the reduced HMPAANa emulsion gel elasticity (G′/G′m) with the oil dispersed phase volume fraction (φ).Palierne’s theoretical model (s) gives a good fitting of our experimental results (b).
Figure 3. Variation of the reduced PAANa emulsion gel elasticity (G′/G′m) with the oil dispersed phase volume fraction (φ). The experimental data (b) are only in qualitative agreement with Palierne’s model (s).
the presence of oil was previously used to immobilize the crystalline structure of emulsions.7 The method allowed us to prepare emulsion gels with oil volume fractions, φ, ranging from 0.25 to 0.65. Samples with φ lower than 0.25 could not be obtained without foam formation. At dispersed phase volume fractions higher than 0.65, the solidlike behavior of the emulsion prevented the homogeneous incorporation of the EDC solution throughout the sample. It is important to remark that neither the presence of oil nor that of hydrophobic alkyl side chains along the polymer backbone prevents the gelation of the matrix. Several difficulties have been encountered in the preparation of emulsion gels formulated with PAANa. This is obviously due to the fact that the PAANa molecules do not adsorb at the oil-water interface and hence do not contribute much to the stabilization of the oil in water emulsion.5 Therefore, the emulsification process was adapted to achieve a good dispersion of oil droplets in the water phase. Both the aqueous (PAANa/amine) and oil (n-dodecane) phases were placed into a flat mould and emulsified in situ using the ultrasonic microprobe at a power of 600 W for 60 s, alternating 5 s bursts and 5 s rest periods. The EDC solution was then added, and the mixture was ultrasonicated for another 20 s under the same conditions as above but at T ) 70 °C to accelerate the gelation reaction kinetics. We also checked by size exclusion chromatography that under these preparation conditions, the linear polymer was not degraded by ultrasonication. With this method of preparation, we were able to obtain emulsion gels with oil volume fractions from 0.05 to 0.2. Rheological Measurements. We have measured the rheological linear response of the emulsion gels. Whatever the emulsion concentration and whatever the interactions between the oil droplets and the elastic matrix, all emulsion gel samples behave like elastic solids. Typically, the domain of linear response extends up to 50% and the loss modulus is always smaller than the elastic modulus by at least 2 orders of magnitude. Moreover, the elastic response did not depend on the frequency over the studied frequency range (0.1-100 Hz). In the following, we report the data collected in the linear regime at a frequency of 1 Hz. The most striking information lies in the fact that the dependence of the reduced elastic modulus (G′/G′m, G′m being the elastic modulus of the gel without oil) on the oil concentration changes drastically when the interactions between the gel matrix and the droplets are varied. In the case of the amphiphilic gel (HMPAANa), the elastic modulus increases with oil concentration (Figure 2),
whereas in the case of the hydrophilic gel (PAANa), it decreases when the droplet concentration increases (Figure 3). Let us first consider the case of amphiphilic gels. We used Palierne’s model11 to give an interpretation of our data. This model describes the rheology of a suspension of deformable droplets in a continuous medium. It has been widely used to successfully describe the rheology of polymer blends.17 This model assumes a homogeneous dispersion of deformable droplets in a viscoelastic matrix. Droplet interfaces are assumed to deform according to the local deformation field of the continuous matrix, which is computed with a mean field hypothesis. In our case, the elastic modulus of the dispersed oil phase is negligible compared to that of the matrix so that the elastic modulus of the gel emulsion is given by11
E(F) 3 dFφ(F) 2 D(F) G′ ) G′m E(F) 1 - dFφ(F) D(F) 1+
∫
∫
(1)
where
γ γ + β′ γ E(F) ) -32G′m2 + 48β′ 2 + 32β′′ 2 + 16 G′m F F F β′ β′′ 32 G′m + 32 G′m (2) F F γ γ + β′ γ D(F) ) 48G′m2 + 48β′ 2 + 32β′′ 2 + 40 G′m + F F F β′ β′′ 64 G′m + 48 G′m (3) F F and
∫φ(F) dF ) φ
(4)
where φ(F) is the volume fraction of droplets with radius F. In the above equations, γ is the interfacial tension, β′ is the bidimensional elastic compression modulus, and β′′ is the elastic shear modulus. The interfacial tension measured at equilibrium is equal to 28 mN m-1, and the interfacial elasticity measurements led to values of β′ and β′′ equal to 24 and 8 mN/m, respectively. φ(F) was measured by quasi-elastic light scattering prior to gelation. (17) Jansseune, T.; Mewis, J.; Moldenaers, P.; Minale, M.; Maffettone, P. J. Non-Newtonian Fluid Mech. 2000, 93, 153-165.
Notes
For obvious reasons, only the droplet size distributions of emulsions stabilized by the HMPAANa molecules could be determined. They were bimodal with 70 ( 10% (in volume) of small droplets with a radius of 0.4 µm (error made on the size of the small droplets is negligible) and 30 ( 10% of droplets with a radius of 14 ( 4 µm. Errors were estimated from the measurements of size distributions of a large number of emulsion samples having the same compositions. We took account of the errors to calculate the storage modulus using eqs 1-4. As shown in Figure 2, our experimental data are in a good agreement with Palierne’s theoretical model, all the more that no adjustable parameter was used to perform the calculation. The PAANa emulsion gels are now considered. Since the PAANa molecules do not adsorb at the droplet interfaces, one may assume that the stress is not transmitted from the gel to the droplets. Consequently, the droplets do not deform but we assume that the gel is free to deform around the droplets. Under this assumption, the previous mean field model (eqs 1-4) leads to11
Langmuir, Vol. 18, No. 16, 2002 6461
G′ ) G′m
1-φ 2 1+ φ 3
(5)
with φ being the oil volume fraction. As shown in Figure 3, the theoretical description does not give an excellent quantitative fitting of our experimental results. We believe that this could be due, at least partly, to the presence of a small amount of air bubbles within the sample. Moreover, in this case the polymer (PAANa) does not adsorb at the interface. Flocculation may thus occur, and the presence of heterogeneities within the emulsion could also explain the disagreement between the experiments and the theoretical model.9 Nevertheless, from a qualitative point of view, it appears that the emulsion gel elasticity depends only on the elasticity of the PAANa gel phase and on the volume fraction of the inclusion. Contrasting to the HMPAANa emulsion gels, it does not depend on the interface rheological properties of the inclusions. LA0200663
